Incandescence is the spontaneous emission of radiation by a hot body. The emission from an idealized "blackbody" is a well understood and described in many physics texts. The output consists of a broad emission whose peak is found at a wavelength given by .lambda.=2.898.times.10.sup.6 /T (nm/K) (see FIG. 1). As a function of wavelength, the emission is asymmetric with approximately 75% of the emission occurring on the long wavelength side of the peak. In addition, the emission is quite broad, particularly on the long wavelength side. Because of this, a blackbody must be heavily filtered at a large cost in efficiency to produce narrowband light.
A clear example of this, is the inefficiency of a blackbody in the production of visible light. If the output of a blackbody is expressed in photometric units, it is found that a temperature of 2600.degree. K is required to obtain a luminous efficacy of 10 lumens/Watt, and a temperature of greater than 3500.degree. K to obtain 40 lm/Watt. By comparison, an ideal narrowband source of green light would have a luminous efficacy as high as 683 lm/W, and an ideal source of white light could have a luminous efficacy of greater than 300 lm/W.
The maximum luminous efficacy for a blackbody is 95 lumen/Watt which occurs at a temperature of 6625.degree. K. Since there are few solid materials which can operate at a temperature above 3000.degree. K, the search for efficient sources of visible incandescence has been primarily a search for materials which can be operated at the highest possible temperatures.
The emission of real materials may be characterized by a spectral emissivity which describes its spectral radiant emittance as a fraction of that of a blackbody at the same temperature. If the emissivity were independent of wavelength, then the emission would have the same wavelength dependence as a blackbody at the same temperature. As an example, tungsten, which is the primary constituent of most visible incandescent sources, has a larger fraction of its emission in the visible than a comparable blackbody. However, at its melting point the luminous efficacy of tungsten is only 53 lm/W, and at practical operating temperatures it has a luminous efficacy in the range of 15-30 lm/W.
Clearly, a great improvement in the efficiency of incandescent lamps for many applications could be achieved if a method could be found to eliminate unwanted emission. For example, B. Hisdal in the Journal of the Optical Society of America, Vol. 52, Page 395, incorporated herein by reference, has calculated that a tungsten filament which has its normal emissivity for wavelengths shorter than 600 nm and an emissivity of zero for wavelengths greater than 600 nm, would give an efficacy of 407 lm/W when operated at 3000.degree. K.
For visible lamps the most successful method developed to date for reducing the unwanted infrared emission consists of surrounding the incandescent source with a selective thermal reflector. This reflector passes visible radiation while reflecting the infrared radiation back onto the filament for reabsorption. This is the working principle of the General Electric IRPAR (Infra-Red Parabolic Aluminum Reflector) lamp (see Photonics Spectra, Page 40, January 1991, which is incorporated herein by reference) which is approximately one-third more efficient than a similar lamp without the reflector. Unfortunately, the practical application of this technique depends on the formation of an image of the filament which is accurately aligned onto the source. Other factors which limit the efficiency gain are the low absorption of the tungsten filament (30%-40%), and practical limits on the transparency, reflectivity and cutoff of the thermal reflector.
Incandescent sources are also characterized by highly divergent emission which is typically almost isotropic. For applications where less divergence is desired, stops, collectors and condensing optics are required. The cost of this optical system frequently exceeds the cost of the lamp which generates the light. In many cases, efficiency must be sacrificed to match the etendue of an optical system.
Optical microcavities used to control the spontaneous emission exist and are described in Physics and Device Applications of Optical Microcavities, H. Yokoyama, 256 Science 66 (1992), Cavity Quantum Electrodynamics, E. A. Hinds, in Advances in Atomic, Molecular, and Optical Physics, eds. D. Bates and B. Bederson, Vol. 28, pp. 237-289 (1991), and in the Jacobsen et al. U.S. Pat. No. 5,469,018, and in U.S. patent application Ser. No. 08/581,632, all of which are incorporated herein by reference. These optical microcavities have the ability to change the decay rate, the directional characteristics and the frequency characteristics of luminescence centers located within them. The study of these phenomena is entitled cavity QED (quantum electrodynamics). Physically, these microcavities have dimensions ranging from less than one wavelength of the emitted light up to tens of wavelengths. Microcavities with semiconductor active layers are being developed as semiconductor lasers and light-emitting diodes (LEDs), and microcavities with phosphor active layers are being developed for display and illumination applications. In all of these devices the efficiency is limited by the low intrinsic efficiency of the semiconductor material or phosphor which generates the light.
Incandescent sources have been formed which are contained within physical microcavities. These are described in the Muller et al. U.S. Pat. No. 5,285,131, Daehler U.S. Pat. No. 4,724,356, and Bloomberg et al. U.S. Pat. No. 5,500,569, all of which are incorporated herein by reference. However, these physical cavities are not designed as optical cavities and exhibit no modification of the spontaneous emission of the incandescent source incorporated.